Many processes have been developed for the formation of amorphous silicon deposited films. They include vacuum deposition process, plasma CVD process, CVD process, reactive sputtering process, ion plating process, and optical CVD process. The most popular among them is the plasma CVD process which has been currently put to practical use.
A disadvantage of forming a deposited film by the aid of plasma is that the substrate surface is damaged by plasma during deposition and this damage adversely affects the deposited film. Therefore, it is important to minimize the damage by plasma particularly in the case where the deposited film is intended to be of high-quality in order to prepare high-performance devices.
To this end, much has been studied and many improvements have been proposed. For example, Japanese Patent Laid-open No. 41047/1985 discloses a process for forming a deposited film on a substrate, without forming plasma, by introducing into the film deposition space the precursors and active species formed in the decomposition space situated separately from the film deposition space. This process is an HR-CVD process. In this HR-CVD process, hydrogen radicals as the active species play an important role in the control of quality and uniformity of a deposited film to be formed, and hence the process needs a means to provide a large amount of hydrogen radicals uniformly to a large area. However, such a means is not mentioned in the above-cited literature.
There has been proposed a process for producing a deposited film by the aid of plasma in sheet-like state. This process is put to practice by the aid of an apparatus for producing a compound semiconductor thin film as disclosed in Japanese Patent Laid-open No. 88820/1984. This apparatus is made up of a means to form sheet plasma containing Group V elements and a means to inject the molecular beam of other elements into the substrate through the sheet plasma.
Japanese Patent Laid-open No. 119676/1986 discloses an apparatus for forming a film which is made up of an unit which serves to form sheet-like plasma of chemically reactive gas or vapor containing elements capable of being the constituents of a deposited film and another unit which serves to introduce a laser beam vertically to a substrate on which a film is to be deposited.
The above-mentioned processes and apparatuses have a disadvantage in common, because they are designed such that plasma is confined to the vicinity of the substrate but is not in contact with the substrate. The disadvantage is that the sheet plasma containing the film constituent elements causes the film constituent elements to deposit on the circumferential walls of the apparatus as well as on the substrate in the sheet plasma forming unit or film forming chamber. This leads to the waste of raw material gas and makes it difficult to form a stable sheet plasma. For these reasons, the prior art does not permit the constantly efficient production of a desired deposited film.